FIG. 1 is a cross-sectional view for illustrating a III-nitride compound semi-conductor light emitting device according to the prior art.
Referring to FIG. 1, the prior III-nitride compound semiconductor light emitting device is fabricated in the following manner. The buffer layer 15 made of undoped GaN, the lower contact layer 20 made of n-GaN, the active layer 30 with a single-quantum-well structure or a multiple-quantum-well structure, and the upper contact layer 40 made of p-GaN, are sequentially deposited on the sapphire substrate 20. Then, mesa etching is performed in such a way to expose the lower contact layer 20. After the electrode layers 50, 60 and 70 to be used as electrodes are formed, the passivation film 80 is formed.
The multiple-quantum-well structure of the active layer 30 is composed of an alternate stack of quantum well layers and quantum barrier layers, in which the quantum well layers may be made of InGaN, and the quantum barrier layers may be InGaN or GaN. It is understood that, if all the quantum well layers and the quantum barrier layers are made of InGaN, the amount of indium (In) in the quantum barrier layers will be lower than the amount of indium in the quantum well layers.
The above-described prior III-nitride compound semiconductor light emitting device has a problem in that the lattice mismatch between the lower contact layer 20 and the active layer 30 is very large. The longer the wavelength of light to be emitted becomes, the larger the lattice mismatch becomes, thus making it more difficult to grow the active layer 30 with high quality. The III-nitride compound semiconductor has the property of piezoelectrics, which becomes stronger as the lattice mismatch becomes larger. Particularly, when the piezoelectric phenomenon occurs within the active layer 30, a change of the energy band shape of the active layer 30 will be caused, resulting in the distortion of the wave functions of holes and electrons, thus reducing light emitting efficiency in the active layer 30.
Meanwhile, as the concentration of n-type dopants (e.g., silicon) in the III-nitride compound semiconductor becomes higher, the thin film quality of the III nitride compound semiconductor layer is deteriorated, and because of its influence, a thin film layer deposited thereon will also be deteriorated in quality. As a result, as the doping concentration of the lower contact layer 20 becomes higher, the thin film quality of the active layer adjacent thereto will be deteriorated, resulting in reductions in the thin film quality of the active layer and the reliability of a device.
As described above, in the prior III-nitride compound semiconductor light emitting device, there are the problem of the lattice mismatch between the lower contact layer 20 and the active layer 30, and the problem of a reduction in the qualities of the lower contact layer 20 and the active layer 30, which is caused by n-type doping to the lower contact layer 20.